The design of a vent pipe typically involves four competing parameters:
a minimal wet time PA1 a maximum heat retention time PA1 minimal outer diameter PA1 flexibility
Often, the advantages of one or more of the above parameters must be sacrificed to achieve the advantages of the other parameters. For example, when flue gases are vented from an appliance through a vent pipe that remains at a temperature below the dew point of the flue gas, the flue gas will condense on the inside surface of the vent pipe. Because the flue gas contains acids, the condensation of the flue gas on the inside of the pipe deposits these acids and contributes to corrosion of the vent pipe. Therefore, it is desired that the inside of the vent pipe have a minimal wet time to help prevent condensation. Wet time is defined as the time for the inner surface of the vent pipe to reach a temperature exceeding the dew point of the flue gas. While a thin walled vent pipe will have a short wet time, a thin wall does not retain heat a sufficient amount of time and the vent pipe will cool down quickly which may result in excessive condensation. Thus, the advantages of a minimal wet time of a thin wall may be offset by the disadvantages of the minimal heat retention time of the thin wall.
As a further example, it is common for gas appliances to have a cycle time of about five minutes. This means that if the vent pipe does not reach a temperature above the dew point of the flue gas before five minutes, the appliance will have cycled off and the flue gas within the vent pipe will begin to condense on the inside of the vent pipe. However, even if the vent pipe does exceed the dew point temperature, it may then quickly cool back down after the appliance cycles off thus allowing condensation to occur. However, if the inside surface of the vent pipe reaches a temperature above the dew point of the flue gas before the appliance cycles off and the vent pipe remains at such a temperature for a sufficient time, the flue gas will exhaust while the vent pipe remains at a temperature above its dew point thereby preventing substantial condensation of the flue gas on the inside surface of the vent pipe.
Double wall vent pipes have been used in an attempt to utilize the air space between the two walls as a type of insulation to insure that the inside surface of the vent pipe heats up quickly and remains above the dew point of the flue gas. However, conventional double wall vent pipes typically have a larger outer diameter and are incapable of being readily bent or flexed to facilitate routing of the pipe through tight clearances. This rigid double wall pipe drives up the expenses of construction by requiring proper routing of the rigid double wall pipe from the flue gas source to an appropriate exhaust point. Further, if a double wall pipe is bent or flexed, the two walls may come in contact with each other and eliminate the space between the two walls and defeat the purpose of the double walls of the vent pipe.
While flexible single wall vent pipe has been used, such vent pipes do not have a sufficient heat retention time to prevent condensation on the inside surface of the vent pipe. Fibrous insulated double wall vent pipes are used; however, such vent pipes have a larger outer diameter which again drives up the cost of construction to insure proper clearances and routing space for the larger diameter pipe.
Thus, a need exists for a vent pipe which is sufficiently flexible to allow the use of the vent pipe in a wide variety of applications while at the same time being of such a structure that the temperature of its inside surface will be maintained above the dew point of the flue gas to prevent condensation within the vent pipe. In particular, there is a need for a vent pipe structured such that its inside surface has a minimal wet time, maximum heat retention time, minimal overall diameter and flexibility.